Silicon semiconductor device,electrode structure therefor, and circuit board mounted therewith

ABSTRACT

A circuit board mounted with a semiconductor device is fabricated by forming on a silicon substrate at least one first metal layer, overlaying a second metal layer to completely cover the first metal layer, covering the whole surface of the second metal layer with an insulating material, etching the insulating material to open a window at a prescribed region of the surface of the second metal layer, selectively imparting adhesiveness to the portion at the window, adhering solder powder to the adhesive portion, melting the solder powder by heating to form a solder bump, selectively imparting adhesiveness to at least one electrode portion of a wiring board, adhering solder powder to the adhesive portion, melting the adhered solder powder by heating to form a solder bump on the electrode portion, and contacting and fusing the solder bump of the silicon substrate and the solder bump of the wiring board so as to form and maintain a prescribed gap between the silicon substrate and the wiring board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrode structure for a siliconsemiconductor, a silicon semiconductor device using the electrodestructure that can be directly bonded to and mounted on a circuit boardwithout use of bonding wire to enable securement of a large gap (bondingheight) between itself and the circuit board at the time of mounting, amethod of fabricating the semiconductor device, a circuit board havingthe semiconductor device mounted thereon, and a method of fabricatingthe circuit board.

2. Description of the prior art

In recent years, electronic technologies have been increasinglyincorporated into virtually every type of device, machine and the likeused in the factory, office and home. This in turn has led to anextensive range of electronic components being used for this purpose.The introduction of electronics into such diverse fields hasparticularly involved the use of very large numbers of circuit boardsmounted with electronic control semiconductor devices. As a result, suchcircuit boards are expanding sharply in range of application, type andnumber.

Although many kinds of practical circuit boards have been developed andput into use, among circuit boards directly mounted with varioussemiconductor devices (ICs) bumps and the like, an especially largepercentage are adopted for reducing electronic equipment size.

When different types of semiconductor devices etc. are mounted on acircuit board, it is necessary and preferable to leave a gap between thecircuit board and the semiconductor device, usually of around 50-100 μm,for injection of sealing resin or to serve as a heat buffer.

Numerous ways have been proposed for providing this gap, which is calledthe "bonding height" in this specification.

These include, for example, the method of establishing the bond betweenthe circuit portion on the printed-circuit board and the deviceelectrodes by use of a conductive bonding agent, the method of bondingthe circuit portion on the printed-circuit board and the chips by fusingsolder bumps formed by plating on the chip side, the method of theplating the electrodes of the printed-circuit board and the deviceelectrodes with copper to the required height, further plating them withgold, and bonding them with eutectic solder, the bonding method by goldball bumps (stud bumps), the bonding method by conductive polymer bumps,the gold-plated resin ball bonding method, the method of bonding byanisotropic conductive film made of microcapsules having insulatingresin balls as cores, and the method of bonding by anisotropicconductive film of the metal-ball-core microcapsule type.

Other methods are also available, such as the gold ball bump directbonding method, but are still too high in cost to be practicallyapplicable.

These conventional methods for establishing bonding height involve anumber of shortcomings. When a conductive resin or other such conductivematerial with relatively high electrical resistance is adopted, theresistance of the bonds is large. On the other hand, when bonding by useof a solder-plated board is adopted, the general practice is to subjectthe semiconductor device side electrodes to activation processingbeforehand. However, this processing is complex and leads to problems ofpoor spread-out of solder wetting and uneven bonding height. It alsoresults in low reliability of the device connections.

A need has therefore been felt for a way of forming solder bumps on asilicon semiconductor device that enables the silicon semiconductordevice to be bonded to a wiring board with high reliability, smallbonding resistance, and freely selectable bonding height.

An object of this invention is therefore to provide an electrodestructure for a silicon semiconductor device formed with solder bumps ofa prescribed height.

Another object of the invention is to provide a solder-bearing siliconsemiconductor device that can be directly bonded to a wiring boardwithout use of bonding wire and is highly reliable and small in bondingresistance, and to provide a method of fabricating the siliconsemiconductor device.

Another object of the invention is to provide a solder-bearing siliconsemiconductor device enabling free selection of the height of solderbumps and to provide a method of fabricating the silicon semiconductordevice.

Another object of the invention is to provide a circuit board readilyformable with solder bumps at the bond portions (electrode portions) andenabling mounting of a semiconductor device at a freely selectablebonding height between itself and the semiconductor device and toprovide a method of fabricating the circuit board.

SUMMARY OF THE INVENTION

To achieve these objects, the invention provides an electrode structurefor a silicon semiconductor device comprising a first metal layerprovided on a semiconductor surface of the semiconductor device to makeohmic contact with the semiconductor and a second metal layer overlaidon the first metal layer to completely cover all surfaces including sidesurfaces thereof, the metal of the second metal layer having higherresistance to corrosion by organic acid and better solder wettabilitythan the metal of the first metal layer.

As the first metal layer there can be used a layer of one memberselected from among Cu, Al, Ti, W and Al-base alloys Al--Si, Al--Ti andAl--W containing not less than 95% of Al. As the second metal layerthere can be used a layer of one member selected from among Cu, Ni andAu but excluding Cu when the first metal layer is of Cu.

The aforesaid electrode structure encompasses the case where anintermediate metal layer of Ni or Cr is further provided between thefirst metal layer and the second metal layer (the intermediate layerbeing limited to Cr when the second metal layer is of Ni), theintermediate metal layer completely covers the first metal layer, andthe second metal layer completely covers the intermediate metal layer.

The aforesaid electrode structure further encompasses the case where allbut a prescribed region of the surface of the second metal layer isshielded by an insulating material.

The invention also provides a solder-bearing silicon semiconductordevice comprising a silicon substrate, at least one first metal layerprovided on the substrate to make ohmic contact with siliconsemiconductor of the substrate and a second metal layer overlaid on thefirst metal layer to completely cover all surfaces including sidesurfaces thereof, the metal of the second metal layer having higherresistance to corrosion by organic acid and better solder wettabilitythan the metal of the first metal layer, insulating material shieldingall but a prescribed region of the surface of the second metal layer,and a solder bump seated on the prescribed region of the surface of thesecond metal layer.

The invention further provides a method of fabricating a solder-bearingsilicon semiconductor device comprising the steps of overlaying at leastone first metal layer on a silicon substrate, overlaying a second metallayer to completely cover the first metal layer, covering the wholesurface of the second metal layer with an insulating material, etchingthe insulating material to open a window at a prescribed region of thesurface of the second metal layer, selectively imparting adhesiveness tothe portion of the second metal layer surface at the window, adheringsolder powder to the formed adhesive portion, and melting the solderpowder by heating to form a solder bump.

The invention further provides a circuit board comprising a wiring boardand a silicon semiconductor device, the silicon semiconductor device andthe wiring board being adhered together by forming a solder bump at atleast one electrode of the silicon semiconductor device by impartingadhesiveness to the surface of the electrode, adhering solder powder tothe adhesive surface and melting the solder powder by heating, forming asolder bump at at least one electrode portion of the wiring board byimparting adhesiveness to the electrode portion, adhering solder powderto the adhesive portion and melting the solder powder by heating, andadhering the solder bump of the silicon semiconductor device and thesolder bump of the wiring board.

The invention further provides a method of fabricating a circuit boardcomprising the steps of selectively imparting adhesiveness to at leastone electrode portion of a wiring board, adhering solder powder to theportion of the wiring board imparted with adhesiveness, melting theadhered solder powder by heating to form a solder bump at the electrodeportion, contacting and fusing the solder bump at the electrode of theaforesaid solder-bearing silicon semiconductor device to the solder bumpof the wiring board so as to form a prescribed gap portion and,optionally, charging insulating resin for sealing into the gap formed.

In the electrode structure for a silicon semiconductor device accordingto the invention, the side in contact with the semiconductor isconstituted of the first metal layer that makes ohmic contact with thesemiconductor and the side provided with the solder bump, which isconstituted of a metal layer having high resistance to corrosion byorganic acid and good solder wettability, completely covers all surfacesof the first metal layer including the side surfaces thereof. Owing tothis configuration, the contact between the semiconductor and theelectrode is excellent, the solder layer can be readily overlaid and thereliability is high. Since the surface of the solder-bearing siliconsemiconductor device to be applied with solder powder is first impartedwith adhesiveness, the height of the solder bump can be easilycontrolled by varying the particle size of the adhered solder powder.The bonding height can therefore be freely selected. Since the siliconsemiconductor device according to the invention is directly mountable onthe wiring board via the solder bump (usually multiple solder bumps),the bonding resistance is low and the reliability high.

In this invention, the bonding portion of the circuit board is similarlyimparted with adhesiveness and adhered with solder powder. Since theheight of the solder bump can therefore be easily controlled, a largebonding height can be secured to further facilitate the bonding heightadjustment.

The above and other features of the present invention will becomeapparent from the following description made with reference to thedrawings.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a sectional view showing an electrode structure for use in asilicon semiconductor device, which is an embodiment of the invention.

FIG. 2 is a sectional view showing an electrode structure for use in asilicon semiconductor device, which is another embodiment of theinvention.

FIG. 3 is a sectional view showing the electrode of the semiconductordevice of FIG. 1 shielded by insulating material.

FIG. 4 is a sectional view showing a window opened in the insulatingmaterial shielding the electrode in FIG. 3 and adhesiveness imparted tothe exposed portion.

FIG. 5 is a sectional view showing solder powder supplied to the windowportion in FIG. 4.

FIG. 6 is a sectional view showing a solder bump formed by heating andmelting the solder powder at the window portion.

FIG. 7 is a sectional view showing a device formed with solder bumpsmountable on a wiring board.

FIG. 8 is a sectional view showing a first embodiment of an electrodestructure formed on a semiconductor chip.

FIG. 9 is a sectional view showing a second embodiment of an electrodestructure formed on a semiconductor chip.

FIG. 10 is a sectional view showing a conventional electrode structureformed on a semiconductor chip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrode structure of the invention for use in a siliconsemiconductor device will be explained first.

As shown in FIG. 1, the electrode structure is provided on the siliconsubstrate surface of a silicon semiconductor device 1 by using vacuumevaporation, sputtering or the like to form a first metal layer 2 to aprescribed thickness and then forming a second metal layer 3 to aprescribed thickness so as to cover the first metal layer 2 completely(including its side surfaces). Although not shown in the drawings,functional layer portions are present in the silicon semiconductor andelectrodes are provided at a prescribed pitch to be positioned at therespective functional layer portions.

The metal used for the first metal layer 2 of the electrode structure isselected from among metals that make ohmic contact with the siliconsubstrate 1. Metals that make ohmic contact with silicon include Al, Cu,Ti, W and Al-base alloys. The Al-base alloys include Al--Si, Al--Ti andAl--W alloys containing not more than 5%, preferably 1-3%, of Si, Ti andW. After the first metal layer 2 has been formed of one of these metalson the silicon semiconductor device, heat treatment is effected toestablish ohmic contact. Al-base alloys have coefficients of thermalexpansion near that of silicon. Use of such an alloy is thereforeadvantageous from the point of alleviating strain at the metal-siliconinterface.

The metal of the second metal layer 3 to be formed on the first metallayer 2 is required to have higher resistance to corrosion by organicacid and better solder wettability than the metal of the first metallayer 2 and to exhibit good contact property with both the siliconsemiconductor and the first metal layer. Specific examples include Cu,Ni and Au. When the first metal layer is made of Cu, a different metal(metal other than Cu) is selected for the second metal layer. Theselection of the metal of the second metal layer is preferably madetaking into account compatibility with the solder used to mount thesilicon semiconductor device 1 on a wiring board and the contactproperty between the silicon semiconductor and the first metal layer.

The second metal layer 3 is formed by vacuum evaporation, sputtering,plating or the like to completely cover the first metal layer 2 andconstitute an electrode.

Although, as just explained, the electrode formed on the semiconductor 1is composed of the first metal layer 2 and the second metal layer 3,preferably, as shown in FIG. 2, it further includes between the firstand second metal layers an intermediate metal layer 4 of Ni or Cr(limited to Cr if the second metal layer is made of Ni). Theintermediate metal layer 4 is overlaid to completely cover the firstmetal layer 2 and the second metal layer 3 is overlaid to completelycover the intermediate metal layer 4. The intermediate metal layer 4enhances the strength of the second metal layer 3 serving as the surfaceelectrode.

There will next be explained a solder-bearing silicon semiconductordevice obtained by forming a solder layer of a thickness of 50-100 μm orgreater (a "solder bump") on a silicon semiconductor device providedwith the electrode of the foregoing structure. After an electrode hasbeen formed on a surface of the silicon semiconductor 1 as shown inFIGS. 1 and 2, the surface of the semiconductor is covered with aninsulating material 5 such as SiO₂, Si₃ N₄, glass, polyimide or the likeby use of a conventional method such as vacuum evaporation, CVD or thelike. The result is shown in FIG. 3. The portion of the insulatingmaterial over the electrode is then removed by an ordinary method or byuse of a mask to open a window as shown in FIG. 4. Only the exposedportion of the second metal layer at this window is selectively impartedwith an adhesive substance 6. Solder powder is then sprinkled to thewindow portion to adhere a layer of solder powder 7 to the formedadhesive portion 6 as shown in FIG. 5. Next, the solder powder 7 ismelted by heating to form a solder bump 8 in the window as shown in FIG.6.

A particular feature of this invention is that the solder bump is formedto a considerable thickness of 50-100 μm or even greater. To form such asolder bump, the semiconductor device having the first metal layer 2covered by the second metal layer 3 is covered by resist (the insulatingmaterial) in the foregoing manner, the metal electrode (the second metallayer) is exposed by etching in the foregoing manner, the exposed metalportion is selectively imparted with adhesiveness, solder powder isadhered to the adhesive portion, and the solder bump is formed bymelting the solder powder. In this case it suffices for the resistforming the window to have a thickness of around 10 μm.

The height of the solder bump can be adjusted by appropriately selectingthe particle size of the solder powder supplied to the window.

Impartation of adhesiveness to the exposed metal portion is effected bysoaking in or coating with an aqueous solution containing at least onemember among napthotriazole derivative, benzotriazole derivative,imidazole derivative, benzoimidazole derivative, mercaptobenzothiazolederivative, and benzothiazole thiofatty acid derivative. (See U.S. Pat.Nos. 5,556,023 and 5,713,997.)

This treatment is effected under conditions of, for example, atemperature of 30-60° C. and a soaking period of 5 sec-5 min. Theaqueous solution is prepared to contain at least one of napthotriazolederivative, benzotriazole derivative, imidazole derivative,benzoimidazole derivative, mercaptobenzothiazole derivative, andbenzothiazole thiofatty acid derivative at a concentration of 0.05-20 wt%. In particular, it is preferable to use as the solution for impartingsolder adhesiveness one that is a slightly acidic liquid containing50-1000 ppm of copper ion.

Although this solution for imparting adhesiveness is relatively stable,it exhibits strong corrosivity to the Al etc. forming the first metallayer and is highly likely to side-etch an electrode of such a metal. Inaddition, for reasons not yet determined, it results in a high defectiveproduct ratio even when side etching does not occur owing to incompletecoverage. The first metal layer must therefore be completely covered bythe second metal layer as explained in the foregoing.

The foregoing method of shielding one surface of the semiconductordevice throughout with insulating material, forming a window (usuallymultiple windows) in the insulating material, selectively impartingadhesiveness to the portion at the window and adhering solder powder tothis portion provides as advantageous features that no troublesomepositioning steps or the like are required and that the adhesion of thesolder powder can be easily automated. Moreover, unlike the conventionalplating method and the like, which encounter difficulty in forming asolder layer of great thickness, the method of the invention enables athick solder layer to be formed with ease because, even if the requiredthickness cannot be secured by a single cycle of the processing steps,the object can be achieved by once heating the semiconductor deviceadhered with the solder layer to melt part of the crown of the solderlayer and then repeating the steps of the cycle to impart adhesivenessto the solder-coated portion, selectively adhere solder powder to theportion imparted with adhesiveness, and melt the adhered solder powderby heating. In addition to providing a thick layer by a singleprocessing cycle, this method enables a still thicker solder layer to beobtained simply by adhering another solder layer to an earlier formedsolder layer.

The method of forming a solder bump on a semiconductor device explainedin the foregoing can also be applied, as shown in FIG. 7, to form asolder bump 13 (usually multiple solder bumps 13 as in the illustratedcase) on a wiring board 11. Specifically, the wiring board 11 is coatedwith resist, only the bonding portions 12 (electrode portions) of thewiring board 11 are exposed, adhesiveness is imparted to the exposedportions, solder powder is supplied to the adhesive portions and thesolder powder is melted by heating to form the solder bumps 13. Thesolder bumps 8 and 13 of the semiconductor device 1 and the wiring board11 are then brought into contact so as to partially fuse, whereby thesemiconductor device can be mounted at a large bonding height (gap) 14.The gap 14 between the device and the wiring board of the circuit boardobtained by mounting the semiconductor device in the foregoing mannercan, if found necessary or desirable, be charged with insulating resinfor sealing.

When a circuit board having a bonding height of 50-100 μm is to befabricated, the bump height on the semiconductor device side (electrodeheight+solder bump height) is made 50-120 μm and the bump height on thewiring board side (electrode height+bump height) is made 30-100 μm sothat the desired bonding height can be obtained without fail even afterthe bumps are bonded by melting a portion of their solder.

The compositions of the solder powders for the semiconductor device andwiring board sides can be made slightly different to give them differentmelting points. In this case, alloying of the higher melting pointsolder bump with the lower melting point solder bump starts before thehigher melting point solder bump melts. This is advantageous because ithelps to preserve the shape of the bumps and, by this, to maintain thedesired bonding height.

This effect can be obtained, for example, by on the semiconductor deviceside using 80 Pb/20 Sn, 95 Pb/5 Sn or other solder with a higher meltingpoint than Sn--Pb eutectic solder and on the wiring board side usingSn--Pb eutectic solder.

The solder-bearing silicon semiconductor device according to theinvention can thus be mounted without use of bonding wires. Asemiconductor device bonded by bonding wire requires resin to be moldedon stems or the like for protecting the bonding wire, which becomesconsiderably larger than the semiconductor device chip itself and takesup a large area on the circuit board. Since the semiconductor device ofthe invention does not require use of bonding wire, it makes a highlyeffective and advantageous contribution to size reduction.

The fact that a metal with good solder wettability is selected for thesecond metal layer makes mounting easy and ensures reliable bonding. Inparticular, the bonding by solder to solder provides highly conductive,strong bonding that enables direct mounting of the semiconductor deviceon the wiring board and greatly enhances bonding reliability.

Owing to the relative freedom in selecting the bonding height, thecircuit board of the invention can be fabricated to have high toleranceand high height. Because of this, the invention circuit board can avoidthe heat problems encountered in the past even when fabricated usingidentical materials and the same kind of semiconductor device. This alsofacilitates circuit board design.

Examples of the invention will now be explained. The invention is,however, not limited to the specific embodiments described.

EXAMPLE 1

Sixteen first metal layers 2 each composed of Al measuring 90 μm indiameter and 1 μm in thickness were formed on a 5-mm square silicon chip1 at a pitch of 250 μm by vacuum evaporation. Second metal layers 3consisting of Cu were formed by vacuum evaporation to a thickness of 1μm on the Al first metal layers 2 so as to cover all surfaces thereofincluding the side surfaces. By this there were obtained 16 electrodesof the structure shown in FIG. 8 (electrode structure I).

Separately, 16 first metal layers 2 were formed on a silicon chip 1 byvacuum evaporation in the same manner as just described except that Cuwas used instead of Al. Second metal layers 3 consisting of Ni wereformed by vacuum evaporation to a thickness of 3 μm on the Cu firstmetal layers 2 so as to cover all surfaces thereof including the sidesurfaces. By this there were obtained 16 electrodes of the structureshown in FIG. 8 (electrode structure II).

Further, 16 first metal layers 2 were formed on a silicon chip 1 byvacuum evaporation in the same manner except that Al-2% Si alloy wasused instead of Al. Second metal layers 3 consisting of Cu were formedby vacuum evaporation to a thickness of 1 μm on the Al--Si alloy firstmetal layers 2 so as to cover all surfaces thereof including the sidesurfaces. By this there were obtained 16 electrodes of the structureshown in FIG. 8 (electrode structure III).

Further, 16 first metal layers 2 each composed of Al measuring 90 μm indiameter and 1 μm in thickness were formed on a 5-mm square silicon chip1 at a pitch of 250 μm by vacuum evaporation. Second metal layers 3consisting of Cu were formed by vacuum evaporation to a thickness of 1μm on the Al first metal layers 2 so as to cover all surfaces thereofincluding the side surfaces. A SiO₂ layer 5 was then formed to athickness of 10 μm throughout as a shield, whereafter it was etched atprescribed locations to form windows and expose the Cu electrodesurfaces. By this there were obtained 16 electrodes of the structureshown in FIG. 9 (electrode structure IV).

As a comparative example, 16 first metal layers 2 each composed of Almeasuring 90 μm in diameter and 1 μm in thickness were formed on a 5-mmsquare silicon chip 1 at a pitch of 250 μm by vacuum evaporation. Secondmetal layers 3 consisting of Cu were formed by vacuum evaporation to athickness of 1 μm on the Al first metal layers 2 so as to cover onlytheir upper surfaces. By this there were obtained 16 electrodes of thestructure shown in FIG. 10 (electrode structure V).

This comparative example was repeated except that the 16 first metallayers 2 were formed by vacuum evaporation using Cu instead of Al andthe second metal layers 3 were formed of Ni to a thickness of 3 μm byvacuum evaporation so as to cover only the top surfaces of the Cu firstmetal layers 2. By this there were obtained 16 electrodes of thestructure shown in FIG. 10 (electrode structure VI).

The aforesaid six silicon chips provided with electrodes were pretreatedwith aqueous solution of sulfuric acid and were then soaked for 5 min ina 40° C. adhesiveness imparting solution composed of a 2 wt % aqueoussolution of 2-undecylimidazole (formula (1) shown below) adjusted to pHof approximately 4.5 with acetic acid. ##STR1##

The six silicon chips were removed from the solution, washed with waterand dried. Next, 20 Sn/80 Pb solder powder of an average particle sizeof 120 μm was sprinkled over the whole surface of the silicon chipsformed with the electrodes. Excess powder at regions other than theelectrodes was blown off with air. The condition of powder adhesion atthis time is summarized in Table 1.

                  TABLE 1                                                         ______________________________________                                        Sample      Adhesion  Remark                                                  ______________________________________                                        Electrode   Good      Adhesion good throughout                                structure (1)                                                                 Electrode   Good      Adhesion good throughout                                structure (2)                                                                 Electrode   Good      Adhesion good throughout                                structure (3)                                                                 Electrode   Good      Adhesion good throughout                                structure (4)                                                                 ______________________________________                                        Comparative examples                                                          ______________________________________                                        Electrode   Poor      Adhesion impaired by                                    structure (5)         generation of extraneous                                                      matter                                                  Electrode   Fair      Adherence at only a portion of                          structure (6)         electrode                                               ______________________________________                                    

The six silicon chips adhered with solder powder were heated for 30 secin a stream of 170° C. nitrogen gas to fix the solder powder, coatedwith water-soluble flux, preheated to 150° C. in a reflow furnacepassing nitrogen gas containing 500 ppm of oxygen and heated to a reflowtemperature of 300° C. to melt the solder powder. They were then cleanedwith hot water. The state of solder bump formation was examined and thebump height and bump height standard deviation (σ) were measured. Theresults are shown in Table 2. As is clear from this table, the heightsof solder bumps are uniform. Therefore, the chips having the electrodestructures I to IV will be able to be all stably mounted on a wiringboard.

                  TABLE 2                                                         ______________________________________                                                 Shape of            Solder bump                                                                           Standard                                 Sample   solder bumps                                                                             Remark   height (μm)                                                                        deviation (σ)                      ______________________________________                                        Electrode                                                                              Good       Good     103     3.0                                      structure (I)       coating                                                   Electrode                                                                              Good       Good     105     3.1                                      structure (II)      coating                                                   Electrode                                                                              Good       Good     105     3.1                                      structure (III)     coating                                                   Electrode                                                                              Good       Good     102     3.0                                      structure (IV)      coating                                                   ______________________________________                                        Comparative examples                                                          ______________________________________                                        Electrode                                                                              Poor       No       --      --                                       structure (V)       coating                                                   Electrode                                                                              Fair       Some     --      --                                       structure (VI)      coating                                                                       missing                                                   ______________________________________                                    

EXAMPLE 2

Sixteen first metal layers 2 each composed of Al measuring 90 μm indiameter and 1 μm in thickness were formed on a 5-mm square silicondevice substrate 1 at a pitch of 250 μm by vacuum evaporation. Secondmetal layers 3 consisting of Cu were formed by vacuum evaporation to athickness of 1 μm on the Al first metal layers 2 so as to cover allsurfaces thereof including the side surfaces. By this there wereobtained 16 electrodes of the structure shown in FIG. 8. This silicondevice substrate 1 provided with electrodes was pretreated with aqueoussolution of sulfuric acid and then soaked for 5 min in a 40° C.adhesiveness imparting solution composed of a 2 wt % aqueous solution of2-undecylimidazole adjusted to pH of approximately 4.5 with acetic acid.

The substrate was removed from the solution, washed with water, driedand sprinkled with 20 Sn/80 Pb solder powder. Excess powder was blownoff with air.

The silicon device substrate adhered with solder powder in this mannerwas heated for 30 sec in a stream of 170° C. nitrogen gas to fix thesolder powder, coated on the solder with water-soluble flux, preheatedto 150° C. in a reflow furnace passing nitrogen gas containing 500 ppmof oxygen and heated to a reflow temperature of 300° C. to melt thesolder powder. It was then cleaned with hot water to obtain asolder-bearing silicon semiconductor device having approximately 60 μmhigh solder bumps.

The foregoing process was repeated except for sprinkling solder powdersof different grain sizes to obtain solder-bearing silicon semiconductordevices having solder bumps with heights measuring 80 μm, 100 μm and 120μm instead of 60 μm.

Next, the copper circuits of a printed wiring board having 16 copperplated circuits of 90 μm width and 250 μm pitch were exposed over 90μm×90 μm at prescribed locations and the remaining portions were coatedwith resist resin. As in the case of the silicon device, the board wasacid-cleaned with aqueous solution of sulfuric acid and then soaked inan adhesiveness imparting solution adjusted to pH of approximately 4.5with acetic acid to impart adhesiveness to the exposed copper circuitportions.

The printed wiring board was then sprinkled with Sn/37 Pb eutecticsolder powder and excess powder was blown off with air. It was next heattreated to form an approximately 30 μm high solder bump at each coppercircuit.

The foregoing process was repeated except for sprinkling solder powdersof different particle sizes to obtain wiring boards having solder bumpswith heights measuring 60 μm, 80 μm and 100 μm instead of 30 μm.

The solder-bearing silicon devices and the wiring boards provided withsolder bumps in the foregoing manner were coated on their solder bumpswith flux, placed with their solder bumps in contact, preheated to 150°C. in a reflow furnace passing nitrogen gas containing 500 ppm of oxygenand heated to a reflow temperature of 200° C. to melt and bond the tipportions of the solder, thereby obtaining circuit boards having siliconsemiconductor devices mounted thereon (see FIG. 7).

The bonding heights obtained with devices and circuit boards havingsolder bumps of different heights are shown in Table 3.

As is clear from this table, circuit boards having bonding heights inthe range of 40-87 μm can be easily obtained by selecting thecombination of device and wiring board used.

                  TABLE 3                                                         ______________________________________                                        Bump height  Bump height                                                      (Silicon device) (μm)                                                                   (Wiring board) (μm)                                                                      Bonding height (μm)                             ______________________________________                                        60           30            40                                                 80           30            56                                                 100          30            69                                                 120          30            87                                                 60           60            55                                                 60           80            64                                                 60           100           77                                                 ______________________________________                                    

As is clear from the foregoing, the silicon semiconductor deviceaccording to the invention is excellent in solder contact property owingto its good resistance to solder corrosion and good solder wettability.When the intermediate metal layer is introduced between the first metallayer and the second metal layer, moreover, the strength of the surfaceelectrode is enhanced. In addition, since the intermediate metal layerand the second metal layer each completely covers the layer thereunder,the metal of the lower layer is safe from being eaten away by solder orbeing side etched by the chemical for imparting adhesiveness.

Another merit is that the solder bump height can be easily controlled byselecting the particle size of the solder powder adhered in the solderpowder adhering step. When this solder-bearing silicon semiconductordevice is mounted on a wiring board, the mounting is effected after thewiring board has also been formed with similar solder bumps. Since thisenables securement of a large bonding height, it provides higher heatresistance property and greater ease of circuit board design for a givencombination of material and semiconductor device.

Since the invention thus enables semiconductor device mounting withoutuse of bonding wire, it achieves low bonding resistance and high bondingstrength while enjoying the superb reliability of solder-to-solderbonding.

What is claimed is:
 1. An electrode structure for a siliconsemiconductor device comprising:a first metal layer provided on asemiconductor surface of the semiconductor device to make ohmic contactwith the semiconductor and a second metal layer overlaid on the firstmetal layer to completely cover all surfaces including side surfacesthereof, the metal of the second metal layer having higher resistance tocorrosion by organic acid and better solder wettability than the metalof the first metal layer.
 2. An electrode structure according to claim1, wherein the metal of the first metal layer is one member selectedfrom among Cu, Al, Ti, W and Al-base alloy and the metal of the secondmetal layer is one member selected from among Cu, Ni and Au butexcluding Cu when the first metal layer is of Cu.
 3. An electrodestructure according to claim 1, wherein an intermediate metal layer ofNi or Cr is further provided between the first metal layer and thesecond metal layer, provided that the intermediate layer being limitedto Cr when the second metal layer is of Ni, the intermediate metal layercompletely covers the first metal layer, and the second metal layercompletely covers the intermediate metal layer.
 4. An electrodestructure according to claim 2, wherein an intermediate metal layer ofNi or Cr is further provided between the first metal layer and thesecond metal layer, provided that the intermediate layer being limitedto Cr when the second metal layer is of Ni, the intermediate metal layercompletely covers the first metal layer, and the second metal layercompletely covers the intermediate metal layer.
 5. An electrodestructure according to claim 2, wherein the Al-base alloy is one memberselected from among Al--Si, Al--Ti and Al--W containing not more than 5%of Si, Ti or W.
 6. An electrode structure according to claim 1, furthercomprising an insulating material that shields all but a window portionof the surface of the second metal layer.
 7. An electrode structureaccording to claim 6, wherein the insulating material is one memberselected from among SiO₂, Si₃ N₄, glass and polyimide resin.
 8. Asolder-bearing silicon semiconductor device comprising:a siliconsemiconductor device substrate, at least one first metal layer providedon a surface of the substrate to make ohmic contact with thesemiconductor thereof and a second metal layer overlaid on the firstmetal layer to completely cover all surfaces including side surfacesthereof, the metal of the second metal layer having higher resistance tocorrosion by organic acid and better solder wettability than the metalof the first metal layer, and a bump seated on a prescribed region ofthe surface of the second metal layer.
 9. A semiconductor deviceaccording to claim 8, wherein the metal of the first metal layer is onemember selected from among Cu, Al, Ti, W and Al-base alloy and the metalof the second metal layer is one member selected from among Cu, Ni andAu but excluding Cu when the first metal layer is of Cu.
 10. Asemiconductor device according to claim 8, wherein an intermediate metallayer of Ni or Cr is further provided between the first metal layer andthe second metal layer, provided that the intermediate layer beinglimited to Cr when the second metal layer is of Ni, the intermediatemetal layer completely covers the first metal layer, and the secondmetal layer completely covers the intermediate metal layer.
 11. Asemiconductor device according to claim 9, wherein an intermediate metallayer of Ni or Cr is further provided between the first metal layer andthe second metal layer, provided that the intermediate layer beinglimited to Cr when the second metal layer is of Ni, the intermediatemetal layer completely covers the first metal layer, and the secondmetal layer completely covers the intermediate metal layer.
 12. Asemiconductor device according to claim 9, wherein the Al-base alloy isone member selected from among Al--Si, Al--Ti and Al--W containing notmore than 5% of Si, Ti or W.
 13. A semiconductor device according toclaim 8 further comprising an insulating material that surrounds thesolder bump seated on the second metal layer.
 14. A semiconductor deviceaccording to claim 13, wherein the insulating material is one memberselected from among SiO₂, Si₃ N₄, glass and polyimide resin.
 15. Acircuit board mounted with a semiconductor device comprising:a wiringboard and a silicon semiconductor device, the silicon semiconductordevice and the wiring board being adhered together by forming a solderbump at at least one electrode of the silicon semiconductor device byimparting adhesiveness to the surface of the electrode, adhering solderpowder to the adhesive surface and melting the solder powder by heating,forming a bump at at least one electrode portion of the wiring board byimparting adhesiveness to the electrode portion, adhering solder powderto the adhesive portion and melting the solder powder by heating, andfusing the solder bump of the silicon semiconductor device and thesolder bump of the wiring board.
 16. A circuit board according to claim15, wherein the electrode of the silicon semiconductor device comprisesa first metal layer making ohmic contact with the semiconductor and asecond metal layer overlaid on the first metal layer to completely coverall surfaces including side surfaces thereof, the second metal layerhaving higher resistance to corrosion by organic acid and better solderwettability than the first metal layer.
 17. A circuit board according toclaim 16, wherein the metal of the first metal layer is one memberselected from among Cu, Al, Ti, W and Al-base alloy and the metal of thesecond metal layer is one member selected from among Cu, Ni and Au butexcluding Cu when the first metal layer is of Cu.
 18. A circuit boardaccording to claim 17, wherein the Al-base alloy is one member selectedfrom among Al--Si, Al--Ti and Al--W containing not more than 5% of Si,Ti or W.